Of recent years increased knowledge has elaborated exact tests, which ensure the absence of these impurities. Nevertheless, anæsthetists of wide experience have obtained results which could not be reconciled with the use of pure chloroform. It has been observed that different chloroforms, all of which answer the official tests for purity, give effects which are difficult to harmonise, and the interpretation of which only appears satisfactory on the assumption that the chloroforms differ in composition. Whilst one chloroform acts most satisfactorily, another produces, during the early stages of administration, a marked excitement and an irregularity of breathing, which prolongs the period of induction. Further investigation has therefore been deemed necessary, and a comprehensive and careful research has elucidated the cause of these hitherto unexplained phenomena (Wade and Finnemore, “Journal of the Chemical Society,” 1904, 85, 938). In the chloroforms which produced anæsthesia in a satisfactory manner, has been demonstrated the presence of ethyl chloride in minute and varying quantities. When the undesirable effects were noted, no ethyl chloride was detected in the anæsthetic. A physiological test conclusively proved that ethyl chloride was the factor which determined these differences. [Sidenote: Value of the investigation] A chloroform which had previously given undesirable effects, and in which the presence of ethyl chloride could not be demonstrated, was modified so as to contain a small proportion of the latter. The chloroform then proved a most satisfactory anæsthetic, and there was entire absence of the excitement and respiratory irregularity previously observed. The results of this research are of the utmost value. In the initial stages of the induction of chloroform anæsthesia, the presence of a small quantity of ethyl chloride has a beneficial effect, leading to the absence of mental excitement, and steadies the breathing. The respiration is stimulated and becomes regular and deep. In these circumstances, satisfactory anæsthesia is induced with rapidity and ease. A CHRONOLOGICAL TABLE OF CHIEF EVENTS AND DISCOVERIES IN THE HISTORY OF ANÆSTHESIA NITROUS OXIDE Joseph Priestley 1776 Humphry Davy 1800 Horace Wells (Colton, Riggs, Evans, Best) 1844 ALCOHOL Collier 1835–42 ETHER